Cinnamic acid can be selectively separated from p-methoxycinnamic acid by facilitated pertraction with Amberlite LA-2 dissolved in dichloromethane, using a U-shaped pertraction cell which allows obtaining the free liquid membrane. The pertraction has been analyzed by means of initial and final mass flows, permeability and selectivity factors. The main factors which control the pertraction selectivity were identified to be the pH-gradient between the feed and stripping phase and the mixing intensity of the aqueous phases. The maximum selectivity factor has been recorded for pH = 2 of feed phase, pH = 8 of stripping phase, rotation speed lower than 300 rpm, and carrier concentration higher than 40 g/1.